This invention relates mass spectrometry and an electrospray ionization source for use in a mass spectrometer which can provide desirable potential distributions for ion generation and/or ion collection.
Electrospray ionization (ESI) introduced by Yamashita et al. Phys. Chem. 88, p4451, (1984), and Fenn et al., Science 246, p64, (1989) is an important method for generating ions for mass spectrometric analysis, and the technique and some of its merits have been particularly described by Smith et al, Anal. Chem., 60, p436, (1988), and others. The merits of ESI include its ability to produce ions from a wide variety of samples such as drugs and biopolymers. Also, ESI is an ideal technique for coupling other chemical separation methods, such as high performance liquid chromatograph (HPLC) or capillary electrophoresis (CE) with a mass spectrometer since ESI can transfer the sample from the liquid phase into the gas phase as required for mass spectrometric analysis.
For mass analysis of ions produced by ESI, almost any type of mass spectrometer can be employed, but time-of-flight mass spectrometers (TOFMS) are widely used, as discussed by Cotter, R. J., Anal. Chem.64, No. 21, p1027A-1039A, (1992). TOFMS is capable of detecting transient signals produced by chromatographic methods because of its high speed of data acquisition. The advantage of using TOFMS for ESI includes unlimited mass range and accurate mass determination which is required for analyzing relatively large biomolecules ( greater than 1000 dalton).
An ESI source generates ions at atmospheric pressure while a mass spectrometer requires a pressure of generally under 10xe2x88x924 Pascal. Therefore, mass analysis requires introducing the ions into a vacuum chamber. This can be achieved by using an interface plate with a small sampling orifice (typically 0.1 to 2 mm in diameter) between a low pressure chamber of the mass spectrometer and the atmospheric pressure region in which the ESI source operates. As a result, sensitivity of mass detection depends both on ionization efficiency (that is, the number of ions produced by the ESI source) and collection efficiency (that is, the number of ions introduced into vacuum).
A typical conventional ESI source and its operation are illustrated in FIG. 1. Such a source consists of a metal capillary 20 of typically 0.1-0.3 mm in diameter, with a tip 24 held approximately 0.5 to 5 cm (but more usually 1 to 3 cm) away from an electrically grounded circular interface 10 having at its center the sampling orifice 14, such as described by Kabarle et al., Anal. Chem. 65, No. 20, p972A-986A, (1993). A potential difference of between 1 to 5 kV (but more typically 2 to 3 kV) is applied to the capillary by power supply 40 to generate a high electrostatic field (106 to 107 V/m) at the capillary tip 24. A sample liquid carrying the analyte to be analyzed by the mass spectrometer, is delivered to tip 24 through an internal passage 20 from a suitable source (such as from a chromatograph or directly from a sample solution via a liquid flow controller). By applying pressure to the sample in the capillary, the liquid leaves the capillary tip as a small highly electrically charged droplets and further undergoes desolvation and breakdown to form single or multicharged gas phase ions in the form of an ion beam 28a. The ions are then collected by the grounded (or negatively charged) interface plate 10 and led through the orifice 14 into an analyzer of the mass spectrometer. During this operation, the voltage applied to the capillary is held constant. Aspects of construction of ESI sources are described, for example, in U.S. Pat. Nos. 5,838,002; 5,788,166; 5,757,994; RE 35,413; 5,986,258; and others.
The ionization efficiency of ESI is dependent on the electric field formed at the capillary tip. This electric field is furthermore a function of the radius of the capillary tip and the voltage applied to the tip. Small capillary tip radius and high voltage generally produce a high field and hence provide high ionization efficiency. Thus, application of a high voltage between the tip and the interface plate is desirable. However, the present invention realizes that in a conventional ESI source such as of FIG. 1, a high electric field near the tip also generates a very strongly convex potential curvature which results in acceleration of the ions away from the center of the beam. Thus, the ion beam in a conventional ESI source is very divergent. Therefore, even though a large number of ions can be generated in such an ion source, only a small portion (1 to 5%) of these ions can be collected for mass spectrometric analysis because of the small size of the interface orifice and the large ion beam divergence. As a result, the sensitivity of mass analysis is sacrificed.
It would be desirable then if a high field could be provided near the capillary tip of an ESI source, but without causing undue beam divergence. It would also be desirable if a means could be provided in an ESI source to at least reduce beam divergence which might otherwise occur. It would further be desirable if both the foregoing high field and divergence corrections could be provided together in the same ESI source of relatively simple construction.
The present invention then, provides methods in which ions are provided to a mass spectrometer having an interface member with an orifice through which ions are received for analysis. A fluid is provided at a capillary tip of an electrospray ionization source. First and second electric fields are alternately provided between the tip and the interface member, the first field directing an ion beam from the tip toward and through the orifice, and the second field decreasing beam divergence from at least part way toward the interface member. That is, beam divergence is decreased from what would otherwise result without use of the second field (that is, versus continuous application of the first field). The first and second fields may be provided at a frequency such that the second field is provided when ions emitted from the tip during provision of the first field are intermediate the tip and interface member. In one aspect the mass spectrometer is a time of flight mass spectrometer which moves ions to be analyzed into an analyzing section of the spectrometer at a predetermined pulse rate. In this case the frequency and pulse rate may be synchronized such that a higher ion current is provided to the analyzing section than would be provided if the first field was continuously applied.
In another aspect, an auxiliary electrode may be used to establish or help establish the first and/or second fields. In this aspect, a potential difference between the tip and the interface member is provided so as to direct an ion beam from the tip toward and through the orifice. A field enhancing potential is applied at the auxiliary electrode so as to increase the electric field gradient from the capillary tip at least part way toward the interface member. In another aspect, a focussing potential is applied at an auxiliary electrode so as to decrease beam divergence from at least part way toward the interface member. In another aspect, the same auxiliary electrode provides both the field enhancing and focussing functions by alternately applying the field enhancing and focussing potentials to an auxiliary electrode. Optionally, either the field enhancing or focussing potentials may be provided intermittently at a predetermined frequency. For example, the field enhancing and focussing potentials may be applied alternately at a frequency such that the focussing potential is applied when ions emitted from the tip during application of the field enhancing potential are intermediate the tip and interface member. In the particular case where the mass spectrometer is a time-of-flight mass spectrometer which moves ions to be analyzed into an analyzing section of the spectrometer at a predetermined pulse rate (just as by a suitable pulser), the frequency and pulse rate may be synchronized such that a higher ion current is provided to the analyzing section than would be provided without the use of the auxiliary electrode.
In yet another aspect of the invention, an additional acceleration for all the charged particles is provided due to switching from the first to the second field. This acceleration causes a separation between the ions and those incompletely desolvated charged droplets which normally result in signal noise for a mass spectrometer and therefore are undesirable. As a result of additional acceleration, signal-to-noise ratio of the instrument can be enhanced.
In one particular construction, the auxiliary electrode faces the interface member. The auxiliary electrode may be at least partly behind the capillary tip. For efficiently reducing the beam divergence, the dimension, that is the diameter of the auxiliary electrode, may be chosen to be the same (including about the same) as the distance between the spray tip and the interface member, for example typically 0.5 to 5 cm. In one particular configuration the auxiliary electrode may be positioned around an axis of the capillary and extend forward from a position behind the tip toward the interface member. For example, the auxiliary electrode may be in the form of an open ended cone with the apex centered around the capillary behind the tip.
Field enhancing and focussing potentials can be adjusted as desired for the particular configuration. For example, for field enhancement the potential may be lower than the tip potential when positive ions are being provided and higher than the tip potential when negative ions are being provided. As to the focussing potential, this may be higher than the tip potential when positive ions are being provided and lower than the tip potential when negative ions are being provided. As to the amount of field enhancement, this may be an increase in potential gradient within 0.5 mm of the capillary tip, of at least 10% or at least 20% or 50%. As to the amount by which divergence of the beam is decreased, this may be such that at least 10% more, or at least 20% more (or even at least 50% more) of the generated ions pass through the interface member orifice during one field switching period.
The present invention further provides an electrospray ionization source for use with a mass spectrometer having an interface member with an orifice. The source includes a capillary and optionally the auxiliary electrode, both positioned and operable in any of the manners as described above. In particular, the field enhancing and/or focussing potentials may be applied to the auxiliary electrode so as to provide the effects described above. The invention further provides a mass spectrometer which includes the interface member and electrospray ionization source, both as previously described, as well as an analyzer section which receives ions from the source which have passed through the orifice, and detects them as a function of mass and charge. A power supply may be included to provide any of the previously described potentials to the electrospray ionization source. In the case where the different fields are to be provided at the above described frequency, the mass spectrometer may optionally include a processor which selects the frequency as a function of mass of the ions to be analyzed by the mass spectrometer.
The various aspects of the present invention can provide any one or more of the following and/or other useful benefits. For example, a high field can be provided near the capillary tip without causing undue beam divergence. The invention can also provide a means to at least reduce beam divergence which might otherwise occur. The invention can further improve the signal-to noise ratio by means of charge separation due to field switching. Both the high field and divergence corrections can be provided together in the same ESI source of simple construction.